Zeolite Composition and Structure

A very brief discussion of the structure and chemistry of zeolites relevant to this review is now given. For a more comprehensive discussion of this subject the reader is referred to an excellent book by Breck (8). 

Zeolites are crystalline aluminosilicates (synthetic and natural), with a chemical composition which corresponds to the general formula 

Type Unit-cell composition Void volume (ml/ml) Pore diameter (A) Thermal decomposition temp.6 (SC) Si/AI ratio 

Zeolites A, X, and Y all consist of tetrahedra linked to form cubo-octahedra or so-called sodalite cage units. When these units are linked through four-membered rings zeolite A is formed (see Fig. 1), whereas linking via the six-membered rings results in zeolites X and Y (see Fig. 2). The latter two zeolites only differ in the Si/Al ratio. 

Mordenite has a channel-like pore structure in which the basic building blocks consist of five-membered rings. A view of the mordenite structure perpendicular to the main channels is shown in Fig. 3. 

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The zeolite composition and structure, which can affect hydrogen transfer activity, are important parameters determining the activity, selectivity, and stability of the zeolite during isobutane alkylation. In the case of USY zeolites, a maximum initial 2-butene conversion was observed for a framework Si/Al ratio of about 6 (63). However, the TMP/DMH ratio, which can be taken as a measure of the alkylation/oligomerization ratio, continuously increased when decreasing the framework Si/Al ratio. On the other hand, the amount and nature of extraframework Al (EFAL) species also affected the alkylation properties of USY zeolites (64). 

Covalent Interactions in Zeolites The Influence of Zeolite Composition and Structure on Acid Softness and Hardness... 

Corma A Covalent interactions in zeoiites the influence of zeolite composition and structure on acid softness and hardness. Stud Surf Sci Catal 1995, 94 736-747. 

Overall the period since the 1980s can be described as a period of explosion in the discovery of new compositions and structures of molecular sieves. This can perhaps be seen most vividly by comparing the numbers of structure types contained in the various editions of the Atlas of Zeolite Structure Types [4]. The first edition (1978) contained 38 structure types, the second edition (1987) 64, the third edition (1992) 85 and the most recent edition (2007) 176. Thus 112 new structure types have been discovered since 1978. However, the reader should be cautioned that a significant number of the structure types included in the Atlas are not truly microporous or molecular sieve materials (i.e., they are not stable for the removal of as-synthesized guest species, typically water or organic templates) and therefore carmot reversibly adsorb molecules or carry out catalytic reactions. Unfortunately, the Atlas gives only limited information on the stability of the structures described. 

Adsorptive separation is a powerful technology in industrial separations. In many cases, adsorption is the only technology available to separate products from industrial process streams when other conventional separation tools fail, such as distillation, absorption, membrane, crystallization and extraction. Itis also demonstrated that zeolites are unique as an adsorbent in adsorptive separation processes. This is because zeolites are crystalline soUds that are composed of many framework structures. Zeolites also have uniform pore openings, ion exchange abiUty and a variety of chemical compositions and crystal particle sizes. With the features mentioned, the degree of zeoUte adsorption is almost infinite. It is also noted that because of the unique characteristics of zeoHtes, such as various pore openings, chemical compositions and structures, many adsorption mechanisms are in existence and are practiced commercially. 

The discrepancy in numbers between natural and synthetic varieties is an expression of the usefulness of zeolitic materials in industry, a reflection of their unique physicochemical properties. The crystal chemistry of these aluminosilicates provides selective absorbtion and exchange of a remarkably wide range of molecules. Some zeolites have been called molecular sieves. This property is exploited in the purification and separation of various chemicals, such as in obtaining gasoline from crude petroleum, pollution control, or radioactive waste disposal (Mumpton, 1978). The synthesis of zeolites with a particular crystal structure, and thus specific absorbtion characteristics, has become very competitive (Fox, 1985). Small, often barely detectable, changes in composition and structure are now covered by patents. A brief review of the crystal chemistry of this mineral group illustrates their potential and introduces those that occur as fibers. 

Table 2.7 Composition and Structural Characteristics of Some Fibrous Zeolites... 

The term fibrous, as applied to zeolites, does not necessarily indicate flexibility. Flexibility, however, is one of the physical properties of cotton stone found on the Isle of Skye, Scotland. This locality provides a feathery sample of mesolite, (Na2Ca2Al6SigO30 8H2O), whose composition and structure place it in the same group as natrolite. The similarity between appearance of this sample and that of the mountain flax variety of palygorskite emphasizes the difficulty of identifying fibrous minerals based on morphology alone. 

The cation plays a prominent structure-directing role in zeolite crystallization. The unique structural characteristics of zeolite frameworks containing polyhedral cages (62, 63) have led to the postulate that the cation stabilizes the formation of structural subunits which are the precursors or nucleating species in crystallization. The many zeolite compositions and complex cation base systems studied allow a test of the structuredirecting role of the cation and the cation templating concept. Table I summarizes the cation base systems from which zeolites have been synthesized. The systems used before 1969 are indicated to illustrate the number and complexities of new cation systems investigated since that time. Table II presents a summary of zeolite framework structure types, the cation systems in which they have been formed, and a proposal for a cation specificity for the formation of each framework type. A similar... 

Figure 3. Overtone and combination band spectrum oj ethylene and water adsorbed on Mn"A zeolite. The ethylene bands lie close to the gaseous (V -f- vn), (vt + vs), 2vlu and 2vs vibrational modes, indicating that the ethylene molecule has retained its chemical composition and structural integrity (-, 1) MnA 4- ethylene (-------------------,2) MnA hydrated CtHt (g) bands.

A typical zeolite synthesis involves mixing together silicate and aluminate solutions or sols to form an aluminosilicate gel, usually instantaneously, which is then treated hydrothermally to give the crystalline product. The composition and structure of the aluminosilicate gel are of considerable interest and characterization of the aluminosilicate species present would give insight into the crystallization process. 

Rollmann and Walsh (266) have recently shown that for a wide variety of zeolites there is a good correlation between shape-selective behavior, as measured by the relative rates of conversion of n-hexane and 3-methyl-pentane, and the rate of coke formation (see Fig. 24). This correlation was considered to provide good evidence that intracrystalline coking is itself a shape-selective reaction. Thus, the rather constrained ZSM-5 pore structure exhibits high shape selectivity, probably via a restricted transition-state mechanism (242b), and therefore has a low rate of coke formation. Zeolite composition and crystal size, although influencing coke formation, were found to be of secondary importance. This type of information is clearly... 

Acidity of OMS and OL materials can be controlled by varying the composition and structure. This is not unexpected based on studies of zeolites and clays. Tremendous adsorptive capacities of OMS and OL materials are observed68,69 on the same order of magnitude of natural manganese nodules. The OMS and OL materials rival zeolitic uptakes and can be as high as 20 g adsorbate per 100 g OMS/OL material. Similar observations53,81 have been made for the natural... 

The actual pH of the exchange solution in contact with a zeolite sample is often essential in determining the result of the exchange experiment, that is the composition and structure within the intrazeolitic volume. For example, an increase in pH of less than two units is sufficient to increase the Pb " " or Cd " " content of zeolite A by about 50% as many hydroxide ions are accepted by the zeolite. This pH may change during ion exchange as hydrogen ions, or alternatively hydroxide ions coordinated to cations, are accepted by the zeolite. 

It is demonstrated that besides ionic interactions, covalent interactions can be important in zeolitic systems. Then, besides acid strength, the concept of acid softness-hardness in zeolites needs to be considered and its importance to explain selectivity effects in orbital controlled reactions is discussed. The influence of zeolite composition and pore structure (electronic confinement) on acid softness-hardness, and orbital control, and their effects on para/ortho selectivity during alkylation of toluene by methanol in large pore zeolites is presented. 

The pH 10 data permit some generalization about the relationship between zeolite composition and stability in HH4NO3. In particular, structural stability decreases as either framework or chemical silica/alumina ratio increases, f hile mechanistic inferences (e gw intracrystalline buffering or "propping" by nonframiework alumina) cannot be drawn from these data, several points are... 

Coke deposition and aging are basic catalyst constraints, constraints lifted in ceitain applications by the discovery of ZSM-5 and related zeolite compositions and pore Structures. It was recognized a number of years ago that coke formation within tiie pores of a zeoUte can be a shape selective reaction (ref. 1). This area has now been excellently reviewed by M. Guisnet and P. Magnoux (ref, 2). 

Describe the chemical compositions and structures of aluminosilicates, clays, and zeolites (Section 22.1, Problems 5-6). 

Zeolite L.—Composition and Structure. Zeolite L was first synthesized by Breck. A typical unit cell content is K9(A102)9(Si02)2 22H2O and Si02/ AI2O3 ratios from 5.2-> 7 can be obtained. The space group is P6mmm with... 

It is too ideal for one Cd4S4 nanocluster to occupy one sodalite cage. In the treatment of zeolite with H2S many mesoporous defects will be produced, and these mesoporous defects may accommodate larger cadmium sulfide clusters. Therefore, in the microporous crystal with a particular structure, there may exist many different cadmium sulfide particles with various sizes. However, the content of these cadmium sulfide clusters located defects is usually limited, and, as a result, they have little effect on the electronic spectral properties of the composite. Detailed composition and structural analysis indicates that the nanoclusters contain not only Cd and S, but also O. In fact, these clusters can be written as Cd4(S,0)4. 

Optical methods for electronic and roto-vibrational transitions and photoelcctron spectroscopy for chemical corc-lcvcl shifts and valence band analysis can yield quantitative information regarding the electronic structure of intrazeolite active centers and their adducts with molecules. Theory and its advancement play a crucial role in the interpretation of the spectra, but practical analysis without extensive theoretical involvement is also feasible. Advances are seen in both the precise applications of spectroscopic methodologies and the synthesis of a variety of novel structures, as well as in examination of systematic patterns of properties that arc governed by the composition and structure of the important zeolite and mesoporous materials. 

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